The Top Five

Five ways to promote healthy soil

Soil has been called the “living skin” of planet Earth, an essential but fragile part of the bio­sphere. As such, it responds negatively to abuse and neglect, and positively to care.

Attention to soil health (or soil quality) has waxed and waned over the years, but it’s on the upswing now. The Washington Tree Fruit Research Commission has rated soil health as a top priority for the past several years. And, the U.S. Department of Agriculture’s Natural Resources Conservation Service is launching a new Soil Health Initiative that focuses on the following principles for improving soil health:

Minimize soil disturbance.

Keep the soil covered as much as possible.

Diversify with crop rotation and cover crops.

Try to provide living roots in the soil for as much of the year as possible.

I find these very easy to understand, to figure out what to do, and to evaluate whether you are making progress. They are not absolutes; rather, they point in a direction and encourage continual movement in that direction. Here are five ways in which I think these principles could apply to orchards:

1. Make your soil a happy home for roots.

Before planting is the time to attend to a number of potential soil quality issues, which can be physical, chemical, or biological in nature. These include:

treating for replant disease and nematodes

dealing with major nutrient problems (such as ­phosphorus or calcium)

adjusting pH (orchards can get pH zonation from higher applications of acidifying nitrogen fertilizers to the tree row versus the alley, or elevated pH from poor quality irrigation water)

increasing organic matter through soil amendments or a cover crop prior to planting (More is generally better; is there a minimum threshold or a maximum not worth pursuing? We don’t know.)

improving drainage

alleviating compaction

These are nuts-and-bolts actions that should all be addressed if needed.

2. Minimize disturbance.

This primarily refers to tillage or cultivation. With a perennial crop like tree fruit, there is obvious disturbance at planting. After that, the soil can go undisturbed for many years, allowing structure to form, food webs to establish, and soil function to improve in general. The main disturbance we might use would be a tillage operation for weed control in the tree row. Keeping any tillage shallow and infrequent is good for the soil and for surface feeder roots of trees.

3. Keep the soil covered as much as possible.

This principle is based on the goals of minimizing soil erosion, which is generally not an issue in orchards, and maintaining surface water infiltration, which can be an issue on some soils. Past research has demonstrated that trees do respond to mulching. How can we make this more feasible to more growers? Practices like mow and blow of alley vegetation onto the tree row, and sweeping flailed prunings onto the tree row can provide a thin mulch layer that may provide a large portion of the mulch benefit at a fraction of the cost of hauling in material. Various fabrics have been tried, some for weed control, some for light reflection, with mixed results on soil.

4. Try to provide living roots in the soil for as much of the year as possible.

Tree root systems, especially those of apples on Malling 9 rootstocks, are not very dense and don’t explore much of the soil volume in the tree row. The soil biota rely on exudates (leakage of carbon-based compounds) from roots for a good portion of their food supply. So our tree rows are generally depauperate in roots. Can we grow other plants with our trees that will benefit the soil, and benefit the orchard? I have tried this with living mulches in the tree row, with mixed results. Competition needs to be controlled at key times, and this is more feasible with established trees. You also need to consider habitat for rodents as a possible undesirable side effect. But this idea has a lot of potential and many opportunities for trying new species, mixtures, and management. This would also address the principle of diversifying the crops in the system. Obviously, crop rotation does not apply much to perennial crops once they are planted. But cover crops can provide a lot of additional diversity. Experience being gained with multispecies mixes (cover crop “cocktails”) is showing promising results that can’t always be explained.

5. Carefully manage your irrigation water.

Water can be a significant source of various minerals that may have beneficial or detrimental effects on soil. Most of our surface water is high quality, while groundwater tends to be more problematic. In addition, irrigation water management is involved in preventing unwanted loss of nutrients below the root zone or cold soil conditions at times when you want the soil biota to increase function. Good water management can also prevent waterlogging and creation of anaerobic conditions in soil leading to disease, denitrification, and other problems.

I think we’ll see more practical ideas on soil quality, and how to measure it, continue to become available. Some will be worthwhile, others not. Most will emerge from field crops settings, and we will have to adapt their use to orchards. The Furrow magazine (published by John Deere) dedicated the entire February 2013, issue to soil quality. It may spur some new ideas for orchards.

For additional information on the NRCS Soil Health ­Initiative, go to www.nrcs.usda.gov/wps/portal/nrcs/main/ national/soils/health/.